Sewage treatment



Dec. 29, 1931. A. M. BUswELL 1,838,475

SEWAG'E TREATMENT I Filed May 9, 1929 Patented Dee. 29, 1931'v `-UNIT-ED sT-ATESPATENT OFFICE 1 THE AUNIVERSITY ILLINOIs ARTHUR u. BUswnLL, or, URBANA, ILLINOIS, AssIoNoI 'ro BOARD or 'musms or 0r ILLINOIS, or URBANA, ILLINOIS, A CORPORATION' or sEwAGE 'rmIA'rInIN'rl vApplication led Ifay 9, 1929. Serial No.. 361,602.

'A I This invention relates to sewage treatment although it is not necessarily limited thereto, and it is'one of the objects of the present invention to provide means for preventing the accumulatlon. of scum or foamdurmg bacteriological fermentatwn or d1 estlon.

The usual method of digesting s udge is toallow 'it to remain'in a tank for three or four months during which time the material is decomposed by bacteriological and chemical y action. The solid matter settles to the bottom .of the tank from which it can be'removed and dried, and the liquid in the tank undergoes decomposition to such an extent that it b'ecomes inoffensive and may' be readily disposed bur-ing the first few daysof decomposition l an appreciable amount of gas having a high scum reached a thickness of twenty-sixl inches in about a month. It had a moisturecontentof 80% and was sostitl' that a shovel was forced into it with di'liculty. l Liquor expressed from the scum had a pI-I of 5. This scum slows down the decompositionprocess and is objectionable in many other respects.

It is an object of the present invention to provide means for preventing the accumulation of scum -in digestion tanks. Since the scum formed is practically'a gel I have found that the liquor from which 'it is separated could be used to soften it'. I pump the liquor .from beneath the scum upward and allow it to flow on to the scum in a gentlelstream or spray. I have found thatyall violent jet or spray action should be avoided since .such ac-A tion results in extensive foam formation. I have found that where the scum has been permitted to accumulate, a ten 'day circulation at the rate of ten gallons per minute completely disintegrated a 26 inch layer of stiff scum and allowed gases evolved :during digestion to escape smoothly into the gas collector. After the scum .has once been softened and disintegrated it `was found that circulation in three minutes the foam level had subsided was rising.

o Lime treatment-is for from five to ten minutes a day prevented anyr further' scum formation.

t sometimes happens that the fermentation of sewagesludge results in the formation of a froth o r'foam rather than Foam- 55 ing appears to be rather spasmodic and of variable intensity. When it occurs in tanks with restrictedA gas vent areas it ma com-l pletely upset the operation ofthe p an. I have seen foam coming'out of the vents of a 00. tank like the froth out of a bottle of warm soda pop. In'tanksequipped with gas co1` lectors the foam will sometimes fill the gas dome, clo the delivery pipe and force its .way out t rough the water seal running all over everything.. The factors which bring about this condition duringsludge digestion do n'ot appear to be the same in all cases. The formation and stability of any foam dependsupon the vis- 7 cosity rather than upon the sur ace tension of the film. The rate or-violence Of foaming will de nd on the rate of gas formation.,

The liquld inthe froth film is more conf centrated than the liquor from which the foam .75 is formed, and I have found that circulating' .the liquor as was done to break up the scum Vwould likewise' dilute and break the foam.

As an experiment a vigorous vfoaming was produced in a tank under Observation by 80 raising the temperature to 37, C., thus greatl increasing the rate of 'gas formation.

Xfter a few hours the foam broke theglas Y seal and flowed over the top of the tank. e circulating pump was then started and withseveninches. Thereafter it was possible to control the foaming by startin the pump whenever the indicator showed t at the level Threeto live minutes circulation'at a time was suicient to break the foam, and routine circulation forf five -to 'ten minutes per day was usually suicient toprevent foam formation.,'. 'M f usually effective in controlling foaming as well as in breaking up the scum, but in aggravated cases the quantity of'limerequire'd is high, amounting in one .ease to $100.00 worth per day. Dilution 1 with water may also be effective in controlling foaming or scum formation but here My method control has the following features 1n its favor.

1. The total costs are not high since the pump lifts against a head of only two or three inches plus the friction in the pipe.

2. The installation costs should not be excessive since with proper connections one small motor and pump could be made to serve a large number of gas vent-s by pumping to one at a time.

3. The method is effective.

The attainment of the above and other objects of the present invention will be apparent from the following specification taken in conjunction with the accompanying drawing forming a part thereof.

The single figure of drawing is a diagram matic sectlon of a sludge digestion tank embodying the principles of my invention.

Sludge is introduced into the digestion tank 1 1n any preferred manner, a. conduit 2 being illustrated for this purpose. The sludge generally consists of about 95% water and 5% of fats, carbohydrates, proteins, and other sewage matter. This is a by far higher concentration than is present in free sewage, since ordinary sewage generally contains about 1/300 of 1% solid matter. This concentration is obtained by passing the sewage through a settling tank, or the like, in the customary manner, before sending ,it into the digestion tank. This is the usual procedure in sewage sludge digestion. In the tank the solid matter gradually settles to the bottom as indicated at 3, the material at 4 being mostly liquid. The sludge is permitted to remain in the tank 1 where it undergoes bacteriological and chemical decomposi:

tion to render it fit for disposition in a known manner. During the first few days of decomposition a considerable amount of gases havmg a high calorilic value are liberated.

The top of the tank is provided with a cover 5, suitably sealed as indicated at 6, thereby permitting the collection of the gases liberated. A pipe 7 extends from the cover 5 to a gas holder. The gases collected have a calorific value about 50% better than that of domestic gas and there is from 1 to 4 cubic feet of gas liberated per day, per capita. Considering the fact that the average amount of gas used is about 2O cubic feet per day, per capita, it may be seen that the amount of gas obtained may be of considerable value.

During the first four to eight days 50% of the solid matter comprising the sludge decomposes to form gases which escape through the pipe 7. New sludge is introduced through the inlet 2 and some of the liquid at the top of the tank is permitted to drain off. The partially decomposed matter at the bottiled of even date herewith, or the slndge may be left in the tank l for complete diges tion in the usual manner. An outlet is provided for withdrawing the completely or partially decomposed material at the bottom of the tank.

As previously stated, a scum or foam tends .to form upon the liquid 4. This scum or foam is indicated at 8. To prevent the accumulation of scum or foam to the extent indicated I provide a eirculator in the form of a pump 9 which withdraws liquid from a level below that to which the scum or foam may extend and redeposits it at the top of the foam or scum. If desired, I may provide a devicefor distributing the returned liquid over the surface at the top of the tank. In the embodiment of my invention herein illustrated this distribution is accomplished by allowing the returned liquid to fall upon two boards 10 nai-led in the form of a cross and fioatingon top of the liquid. This float is provided with a rod 11 extending outside of the tank cover 5 which rod serves as an indicatoil of the level of the surface in the tank.

The discharge pipe 12 is placed within a very short distance, two or three inches, from the member 10, thereby further avoiding violent jet or spray action.

The circulation of liquid as described will (A) soften and dissipate scum which may have accumulated at the top of the tank. (B) Will dilute and dissipate foam which may have accumulated at the top of the tank. (C) Will result in desolation of gases dissolved inthe liquid at ythe level of the intake pipe, by reason of the fact that the pressure upon the liquid is reduced by raising it from a lower to a higher level, which change in pressure lessens the solubility of the dissolved gases.- (D) Will disperse or comminute solid particle of colloidal or gel-like nature rendering them more susceptible to bacterial action. Will prevent the development of local zones in which bacterial bi-products accumulate, and slow down or prevent bacterial action (as the occurrence of acid condition in thick scum). (F) Will allow the operation of a sludge digestion tank without preliminary feeding by or provision for the accumulation of digested sludge. (Current practice requires that 4 to 50 pounds of digested sludge be present in a tank for each pound of fresh sludge added daily.)

In compliance with ,the requirements of the patent statutes I have herein shown and described a preferred embodiment of my invention. It is, however, to be understood that the invention is not limited to the pre- ClSe construction shown, the same being merely illustrative of the invention. What I consider new and desire to secure by Letters Patent is: A 1. A digestion tank including a gas tight cover, means for collecting the gases generated during the digestion process, and means for circulating the liquid of the tank -from a level below that to which the scumor foam may accumulate tothe top of the scum or foam, thereby dissipating any scum or Vfoam that may form.

2. In a digestion tank, means for circulating the liquid of the tank. from a level be- 1 low that to which the scum or foam may accumulate to the top of theScum or foam thereby dissipating any scum or foam that may form.

3. Sludge digestion means comprising a digestion chamber, a` pump for circulating the liquid of the upper portion of the cham` ber for dissipating any scum or foam that may form, and means for collecting the gases generated in the chamber.

4. Sludge digestion means comprising a digestion chamber, a pump for -circulating the liquid of the upper portion of the chamber for dissipatingv any scum or foam that may form, and means for distributingthe circulation of the liquid at the surface of the scum or foam. j

5. The method of preventing the accumu` lation o f scum or foamduring sludge diges'- I tion which comprises withdrawing slomeV of the liquid from below the-depth to which;

35 scum or foam may. form and redepositing it upon the scum or foam formed.

6. Thefmethod of preventing the accumulation of scum or'foam during, sludge digesf tion which comprises dissipating the scum or '40 foamby periodlcally depositing thereon liquid drawn from the digestion chamber at a level below the depth to which the scum has accumulated. 7. In a fermentation or digestion tank the method of preventingthe formation of scum or foam during the decomposition process. which comprises s raying the top of the .liq-

' uid with some of t e liquid from the tank.

8. The method of dssipatin scum* on 50 top of liquid in a receptacle, whic comprises withdrawingliquid from the receptacle at a' level below that at which the scum forms and depositin it upon the scum.v

9. In t e digestion of organic matter, the' method of operation which lncludes the stepof withdrawin liquor from belowthe top v surface of the 'gesting matter and deposit'- v ing it on top thereof. v i In witness whereof, I hereuntoy subscribe m my name this 3rd da of Ma A. D'.'1929.

. AR isc-iaUsw-ELL. 

